The present invention relates to a discharge lamp driver circuit.
High intensity discharge lamps (HID lamps) are arc lamps and do not have a physical filament as is the case with incandescent lamps. The HID lamp comprises a sealed envelope filled with xenon gas at a pressure above one atmosphere. The active volume of the lamp also includes within it mercury and other compounds (salts) which emit an intense light once an arc is established within the active volume of the lamp.
An HID lamp is approximately four time more efficient than alternative halogen lamps and produces a colour temperature similar to that of natural daylight. HID lamps are used for example in premium quality cars, floodlighting, architectural lighting and street lighting.
In order to light an HID lamp different conditions are required to initiate and then to maintain an arc. Firstly, a high voltage, typically up to 25000 volts, is required to establish the arc. Once the arc has been established, a power source must be connected to the lamp so as to maintain the arc. In conventional HID lamp driver circuits, the power source consists of two stages, that is a first stage to provide a high DC value and a second stage to convert this DC value into an AC voltage to maintain the arc. It is desirable to stabilise the power supply to the lamp by controlling the supplied voltage and/or current, and in order to achieve this conventional circuits require some form of computational capability so as to produce an output representing the product of the supplied voltage and current. As a result the conventional HID lamp driver circuits are relatively complex and expensive.